gistfile1.txt

protected function downloadFile(ServerRequestInterface $request, string $filePath)
    {
        $fileName = basename($filePath);
        $contentType = (new \finfo(FILEINFO_MIME))->file($filePath);

        if (! file_exists($filePath)) {
            throw new \Exception("File not found: $filePath");
        }

        if (! is_readable($filePath)) {
            throw new \Exception("File not readable: $filePath");
        }

        // remove headers that might unnecessarily clutter up the output
        $response = (new Response())
            ->withoutHeader('Cache-Control')
            ->withoutHeader('Pragma');

        // default action is to send the entire file
        $byteOffset = 0;
        $byteLength = $fileSize = filesize($filePath);

        $response = $response
            ->withHeader('Accept-Ranges', 'bytes')
            ->withHeader('Content-Disposition', "attachment; filename=$fileName");

        $server = $request->getServerParams();

        if (isset($server['HTTP_RANGE']) && preg_match('%bytes=(\d+)-(\d+)?%i', $server['HTTP_RANGE'], $match)) {
            $byteOffset = (int) $match[1];
            if (isset($match[2])) {
                $finishBytes = (int) $match[2];
                $byteLength  = $finishBytes + 1;
            } else {
                $finishBytes = $fileSize - 1;
            }
            $response = $response
                ->withStatus(206, 'Partial Content')
                ->withHeader('Content-Range', "bytes {$byteOffset}-{$finishBytes}/{$fileSize}");
        }

        $byteRange = $byteLength - $byteOffset;

        $response  = $response
            ->withHeader('Content-Type', $contentType)
            ->withHeader('Content-Length', $byteRange)
            ->withHeader('Content-Description', 'Panoramic download')
            ->withHeader('Pragma', 'public')
            ->withHeader('Expires', '0')
            ->withHeader('Cache-Control', 'must-revalidate');

        $bufferSize = 512 * 16;
        $bytePool   = $byteRange;

        if (! $fh = fopen($filePath, 'r')) {
            throw new \Exception("Could not get filehandler for reading: $filePath");
        }

        $buffer = new RelativeStream(new Stream($filePath, 'r'), $byteOffset);

//        if (fseek($fh, $byteOffset, SEEK_SET) == -1) {
//            throw new \Exception("Could not seek to offset $byteOffset in file: $filePath");
//        }

        $buffer->seek($byteOffset);

        while ($bytePool > 0) {
            $chunkSizeRequested = min($bufferSize, $bytePool);
            //$buffer->attach(fread($fh, $chunkSizeRequested));
            $buffer->seek($chunkSizeRequested, SEEK_CUR);
            
            
//            $buffer->seek($byteOffset, SEEK_SET);
            $chunkSizeActual = strlen($buffer);

            if ($chunkSizeActual == 0) {
                throw new \Exception("Chunksize became 0");
            }

            $bytePool -= $chunkSizeActual;
        }

        return $response->withBody($buffer);
}